Donald Trump called on Attorney General Jeff Sessions to halt Special Counsel Robert Mueller’s investigation of Russian interference in the 2016 election, ramping up his attacks on the probe as the president’s former campaign chairman goes on trial for unrelated criminal charges.

“This is a terrible situation and Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now, before it continues to stain our country any further,” Trump said Wednesday in a Twitter posting. “Bob Mueller is totally conflicted, and his 17 Angry Democrats that are doing his dirty work are a disgrace to USA!”

The public request to his attorney general to stop a federal investigation is an extraordinary departure from traditional boundaries between the president and law enforcement. Presidents typically avoid public comment on ongoing criminal investigations to avoid any perception they are seeking to influence the outcome.

“Jeez,” gasped Republican Senator Susan Collins of Maine as she was read the president’s tweet in an interview. “This is unbelievable.”

“Those comments are totally inappropriate,” she added. “The president should not be talking about the investigation at all.”

Sessions, who has recused himself from supervising the Mueller investigation, didn’t immediately respond to the president’s tweet. Sarah Isgur Flores, a spokeswoman for the Justice Department, declined to comment.

Trump’s tweet was also immediately condemned by some Democratic lawmakers as a blatant attempt to obstruct justice.

“The President of the United States just called on his Attorney General to put an end to an investigation in which the President, his family and campaign may be implicated,” Representative Adam Schiff of California, the top Democrat on the House Intelligence Committee, said on Twitter. “This is an attempt to obstruct justice hiding in plain sight. America must never accept it.”

..This is a terrible situation and Attorney General Jeff Sessions should stop this Rigged Witch Hunt right now, before it continues to stain our country any further. Bob Mueller is totally conflicted, and his 17 Angry Democrats that are doing his dirty work are a disgrace to USA!

— Donald J. Trump (@realDonaldTrump) August 1, 2018

The request could reinvigorate legislation that would protect Mueller from being fired without cause. The measure won bipartisan support from the Senate Judiciary Committee in April, but Senate Majority Leader Mitch McConnell has said it won’t get consideration by the full Senate.

In view of the tweet, “I think it would be helpful to have the debate on that bill,” Collins said. “It would send a message to Mr. Mueller that he has strong support in Congress.”

McConnell and House Speaker Paul Ryan have defended Mueller’s investigation in the face of Trump’s attacks but said they don’t believe Trump would fire the special counsel. Lawmakers of both parties have warned Trump that firing Mueller would create a constitutional crisis. Ryan’s spokesman, Doug Andres, said Wednesday that the speaker’s position hasn’t changed.

The president’s message was part of a series of tweets attacking Mueller on the first full day of testimony in Manafort’s tax and bank-fraud trial amid other signs Mueller’s investigation is drawing closer to Trump. Prosecutors are expected to detail Manafort’s lucrative business relationship with Russia-friendly leaders in Ukraine.

It also followed a CNN report last week that the president’s former personal lawyer, Michael Cohen, is prepared to testify that Trump knew in advance about a notorious 2016 meeting between top campaign officials, including his son Donald Trump Jr. and son-in-law Jared Kushner, and Russians who were said to be promising dirt on Hillary Clinton. The president denied the claim in a tweet.

Sessions, an early Trump backer, recused himself from the Russia investigation in March 2017 as controversy grew over his conversations during the presidential campaign with Russia’s ambassador to the U.S. That probe has since swept up officials in Trump’s campaign and his family members.

Trump said last summer he would have chosen a different attorney general had he known Sessions would recuse himself from supervising the investigation of election interference. Trump has periodically launched barrages of public attacks on Sessions related to the special counsel’s investigation.

Mueller’s team included Trump’s past public attacks on Sessions in a list of topics they would like to cover in questioning the president, suggesting that the special counsel is examining whether they were intended to influence the investigation and possibly obstruct justice.

Sessions delegated authority over the election investigation after recusing himself to Deputy Attorney General Rod Rosenstein, who in May 2017 appointed Mueller special counsel to the probe. A group of conservative congressional allies of the president led by Representatives Mark Meadows and Jim Jordan launched an effort last week to impeach Rosenstein.

Bloomberg’s David Voreacos, Andrew Harris, Chris Strohm and Steven T. Dennis contributed.